Compositions and methods for planarizing or polishing the surface of a substrate, especially for chemical-mechanical polishing (CMP), are well known in the art. Polishing compositions (also known as polishing slurries) typically contain an abrasive material in an aqueous solution and are applied to a surface by contacting the surface with a polishing pad saturated with the polishing composition. Typical abrasive materials include silicon dioxide, cerium oxide, aluminum oxide, zirconium oxide, and tin oxide. U.S. Pat. No. 5,527,423, for example, describes a method for chemically-mechanically polishing a metal layer by contacting the surface with a polishing slurry comprising high purity fine metal oxide particles in an aqueous medium. The polishing slurry is typically used in conjunction with a polishing pad (e.g., polishing cloth or disk). Suitable polishing pads are described in U.S. Pat. Nos. 6,062,968, 6,117,000, and 6,126,532, which disclose the use of sintered polyurethane polishing pads having an open-celled porous network, and U.S. Pat. No. 5,489,233, which discloses the use of solid polishing pads having a surface texture or pattern. Alternatively, the abrasive material may be incorporated into the polishing pad. U.S. Pat. No. 5,958,794 discloses a fixed abrasive polishing pad.
Polishing compositions for silicon-based inter-metal dielectric layers have been particularly well developed in the semiconductor industry, and the chemical and mechanical nature of polishing and wear of the silicon-based dielectrics is reasonably well understood. One problem with the silicon-based dielectric materials, however, is that their dielectric constant is relatively high, being approximately 3.9 or higher, depending on factors such as residual moisture content. As a result, the capacitance between the conductive layers is also relatively high, which in turn limits the speed (frequency) at which a circuit can operate. Strategies being developed to reduce the capacitance include (1) incorporating metals with lower resistivity values (e.g., copper), and (2) providing electrical isolation with insulating materials having lower dielectric constants relative to silicon dioxide.
One way to fabricate planar copper circuit traces on a silicon dioxide substrate is referred to as the damascene process. In accordance with this process, the silicon dioxide dielectric surface is patterned by a conventional dry etch process to form holes and trenches for vertical and horizontal interconnects. The patterned surface is coated with an adhesion-promoting layer such as titanium or tantalum and/or a diffusion barrier layer such as titanium nitride or tantalum nitride. The adhesion-promoting layer and/or the diffusion barrier layer are then over-coated with a copper layer. Chemical-mechanical polishing is employed to reduce the thickness of the copper over-layer, as well as the thickness of any adhesion-promoting layer and/or diffusion barrier layer, until a planar surface that exposes elevated portions of the silicon dioxide surface is obtained. The vias and trenches remain filled with electrically conductive copper forming the circuit interconnects.
Previously, it was believed that the removal rate of the copper and the adhesion-promoting layer and/or the diffusion barrier layer must both greatly exceed the removal rate of silicon dioxide so that polishing effectively stops when elevated portions of the silicon dioxide are exposed. The ratio of the removal rate of copper to the removal rate of silicon dioxide base is called “selectivity.” A minimum selectivity of about 50 was desired for such chemical-mechanical polishing. However, when high selectivity copper slurries are used, the copper layers are easily over-polished creating a depression or “dishing” effect in the copper vias and trenches. This feature distortion is unacceptable due to lithographic and other constraints in semiconductor manufacturing. Another feature distortion that is unsuitable for semiconductor manufacturing is called “erosion.” Erosion is the topography difference between a field of silicon oxide and a dense array of copper vias or trenches. In chemical-mechanical polishing, the materials in the dense array are removed or eroded at a faster rate than the surrounding field of silicon oxide. This causes a topography difference between the field of silicon oxide and the dense copper array. The industry standard for erosion is typically less than 500 Angstroms (Å).
A number of systems for chemical-mechanical polishing of copper have been disclosed. Kumar et al. in an article entitled “Chemical-Mechanical Polishing of Copper in Glycerol Based Slurries” (Materials Research Society Symposium Proceedings, 1996) disclose a slurry that contains glycerol and abrasive alumina particles. An article by Gutmann et al. entitled “Chemical-Mechanical Polishing of Copper with Oxide and Polymer Interlevel Dielectrics” (Thin Solid Films, 1995) discloses slurries based on either ammonium hydroxide or nitric acid that may contain benzotriazole (BTA) as an inhibitor of copper dissolution. Luo et al. in an article entitled “Stabilization of Alumina Slurry for Chemical-Mechanical Polishing of Copper” (Langmuir, 1996) discloses alumina-ferric nitrate slurries that contain polymeric surfactants and BTA. Carpio et al. in an article entitled “Initial Study on Copper CMP Slurry Chemistries” (Thin Solid Films, 1995) disclose slurries that contain either alumina or silica particles, nitric acid or ammonium hydroxide, with hydrogen peroxide or potassium permanganate as an oxidizer. While present day chemical-mechanical polishing systems are capable of removing a copper over-layer from a silicon dioxide substrate, the systems do not entirely satisfy the rigorous demands of the semiconductor industry. These requirements can be summarized as follows. First, there is a need for high removal rates of copper to satisfy throughput demands. Secondly, there must be excellent topography uniformity across the substrate. Finally, the CMP method must minimize local dishing and erosion effects to satisfy ever increasing lithographic demands.
The use of surfactants in abrasive compositions for copper polishing has been disclosed. For example, U.S. Pat. No. 6,270,393 discloses an abrasive slurry comprising alumina, an inorganic salt, a water-soluble chelating agent, and a surfactant which purportedly acts as a dispersant for the abrasive. The '393 patent discloses that the surfactant can be a nonionic surfactant having a hydrophilic-lipophilic balance (HLB) value of 10 or greater. U.S. Pat. No. 6,348,076 discloses polishing compositions for metal layer CMP comprising surfactants, in particular anionic surfactants. U.S. Pat. No. 6,375,545 discloses a polishing composition comprising a polymer particle abrasive in combination with an inorganic abrasive, an oxidizer, and an organic acid. U.S. Pat. No. 6,375,693 discloses a polishing composition comprising an oxidizer, a corrosion inhibitor, and an anionic surfactant (e.g., a fatty acid sulfonate ester surfactant). U.S. Pat. No. 6,383,240 discloses an aqueous dispersion for CMP comprising abrasive particles and an amphipathic surfactant having an HLB value of 6 or lower. U.S. Published Patent Application 2001/0008828 A1 discloses an aqueous polishing composition for copper and barrier film polishing comprising an abrasive, an organic acid, a heterocyclic compound, an oxidizer, and optionally a surfactant. U.S. Published Patent Application 2002/0023389 discloses the use of a surfactant, which can be an anionic, cationic, or nonionic surfactant, to minimize erosion and scratching of a substrate surface layer. U.S. Published Patent Application 2002/0037642 discloses a polishing composition for use in copper polishing comprising an oxidizer, a carboxylic acid, and an abrasive containing mainly aggregated θ-alumina particles having an average particle size of about 50 to 500 nm. The abrasive particles can be dispersed using a surfactant dispersing agent selected from anionic, cationic, ampholytic, or nonionic surfactants. WO 01/32794 A1 discloses a CMP slurry, for polishing a substrate with a tantalum barrier layer, comprising an organic additive, which can be any of a variety of surfactants, that purportedly forms bonds with the surface of the silica or copper substrate and suppresses formation of silica precipitates and copper staining. WO 02/04573 A2 discloses a polishing composition comprising hydrogen peroxide that is stabilized in the presence of silica abrasive further comprising an organic acid, benzotriazole, and a surfactant. EP 1 150 341 A1 discloses a polishing composition comprising an abrasive having a particle size of less than 100 nm, an oxidizer, an organic acid, benzotriazole, and a surfactant.
Despite the disclosure of numerous CMP compositions and methods, a need remains for CMP compositions and methods for use in the polishing of substrates containing metal layers, especially copper-containing metal layers. The invention provides such a CMP method, particularly that provides good selectivity of substrate layer removal as well as decreased dishing and erosion. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.